Characterization of HNbMoO6, HNbWO6 and HTiNbO5 as solid acids and their catalytic properties for esterification reaction

Abstract

HTiNbO5, HNbMoO6·H2O and HNbWO6·1.5H2O were prepared by proton-exchange of the precursors KTiNbO5, LiNbMoO6 and LiNbWO6, respectively, which were synthesized by a solid-state reaction method. The morphology and the micro-structure were characterized by means of scanning electron microscope (SEM), transmission electron microscope (TEM), powder X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FT-IR). NH3 temperature-programmed desorption (NH3-TPD) was used to measure the acidic properties of the samples. The Sanderson electronegativity and Kataoka′s bond strength theory were also applied to investigate the acid properties of the as-prepared samples. Finally, their catalytic activities were evaluated through catalyzing the liquid-phase esterification reaction of acetic acid and n-butanol to form n-butyl acetate. It has been revealed that H+ ions existed in different forms and interaction modes with MO bond in the solid acids. The acid strength for these as-prepared samples follows the order HNbMoO6·H2O>HNbWO6·1.5H2O>HTiNbO5. In this work, HNbMoO6·H2O presents an excellent catalytic activity for the esterification reaction, while HNbWO6·1.5H2O and HTiNbO5 have little catalytic activity. The result suggested that the catalytic activity of the layered solid acid for the esterification reaction was mainly controlled by gallery height.